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Too much chlorine in water can damage tree leaves and other tissues. The concentration threshold for tree damage is as low as 0.5 parts per million. The extent of tree damage from chlorine depends on the concentration, duration of exposure, manner in which the tree takes up the chlorine, and season of the year. Chlorine damage is greater when exposure occurs while trees in your yard are actively growing rather than when they are dormant.

Toxicity Symptoms

Chlorine is a trace element essential to tree nutrition, but too much chlorine is poisonous. The leaves of trees are the first structures to show effects of chlorine toxicity. They will show a burned or scorched appearance with brown, dead tissue on the tips and edges, as well as between the leaf veins. Affected leaves may also be stunted, turn yellow and drop off early.

Poisoning Process

Chlorine dissolved in water reacts with soil elements to form chloride compounds taken up by tree roots. Excess chlorides lead to the symptoms of chlorine poisoning. Chlorine enters soil from swimming pool water runoff, water from heavily salted roads and driveways, chlorine from air pollution that dissolves in rainwater, and naturally occurring chloride salts in the soil that dissolve in irrigation water or rainwater. Chlorine levels in municipal drinking water generally are too low to harm trees, but as a general rule, if you can smell chlorine in the water, don’t use it to water your trees. The human nose can smell chlorine in concentrations as low as 1 part per million.

Susceptible Trees

Certain tree species are particularly sensitive to chlorine poisoning. Among them are most varieties of maple tree and box elder trees. Also chlorine-sensitive are ash, crabapple, dogwood, horse chestnut, mulberry, pin oak, sweet gum and yellowwood.

Tree Treatments

Treat trees damaged by waterborne chlorine by irrigating with plenty of clean, chlorine-free water. This clean water will dilute and carry away chlorides from the soil and plant tissues. If your only water source is heavily chlorinated, remove the chlorine by putting your water in a container in a sunny spot for a week before using it on trees. If your soil is heavily laden with chlorides, apply gypsum to extract the chlorine. Spread the gypsum at the rate of 58 pounds per 1,000 square feet and work it into the top soil. Water thoroughly to leach the toxic levels of chlorine from the soil.

About the Author

Herb Kirchhoff has more than three decades of hands-on experience as an avid garden hobbyist and home handyman. Since retiring from the news business in 2008, Kirchhoff takes care of a 12-acre rural Michigan lakefront property and applies his experience to his vegetable and flower gardens and home repair and renovation projects.